Clay Mineralogical Characteristics Of The Cenozoic Sediments From The Northern Qinghai-Tibetan Plateau:Indictors For Tectonic And Climatic Evolution

The Qinghai-Tibetan Plateau (QTP) is the highest, largest, thickest, and most voluminal plateau, which is honoured as ’the rooftops of the world’ and has significantly uplifted since Cenzoic. The collision between the India-Eurasian plates is regarded as a main factor that results in remarkably multi-phase uplifts. Due to the India-Eurasian collision, basins in the north of QTP drifted further north, the Neo-Tethys Sea retreated from eastern to western Tarim, and the QTP uplifted from south to north gradually and differently. The India-Eurasian collision and its’long-term effects’significantly changed the structural framework of the northern QTP, even Asia. Meanwhile, the QTP could have changed the atmospheric circulation when it achieved a certain height and scale, inducing the formation of the East Aisan Monsoon and affecting its evolution as well as the pattern of westlies. It could obstruct the moisture of Indian Ocean from inputing inland of the QTP. Persistent extrusion caused the retreat of the Neo-Tethys Sea and northward drift of the QTP, which significantly changed climatic patterns within and around the QTP regionally even globally. Therefore, understanding the uplift of QTP and the climatic evolution can benefit us to know how the uplift of QTP and its domino offect impact the climate around the QTP. Meanwhile, this will also enlarge our knowlege of science to know couple effects among tectonic movement, uplift of QTP, and climatic change. The formation, uplift, and its climatic effect have been and will be international and frontier topics in geology. Clay mineralogical method was employed to study the clay mineral phases and their aboundances from representative Cenzoic basin sediments, such as Tarim Basin, Qaidam Basin, and Gonghe Basin. Clay mineral morphology was also documented and local and global climate proxies were compared to discuss the paleoclimatic evolution and their potentially driving mechanisms in the north part of Qinghai-Tibet Plateau since Cenzoic.The Tarim Basin located in the northwest of QTP documents a continuous Cenzoic sedimentary sequence from a marine to terrestrial deposit. Clay mineralogical and bulk mineralogical assemblage reveal a whole climatic and tectonic evolution in the Tarim area. Clay mineralogy analyses were carried out on Cenozoic sedimentary rocks collected from the southwestern margin of the Tarim Basin, northwest China, using X-ray diffraction (XRD) and scanning electronmicroscopy (SEM) methods. These investigations were aimed at determining the paleoclimate of the area in early Cenozoic times, especially during the Eocene to early Oligocene. The dramatic declines of kaolinite and smectite, increase of non-clay mineral gypsum and transition of clay mineral mopgology indicate the paleoclimate significantly underwent a change from warm and humid to cool and dry at the late Eocene in the Tarim area. This climate transition documented by the clay mineralogical study in the Tarim Basin records in the QTP and is timely consistent with the effects of the retreat of the Neo-Tethys Sea. The retreat of the Neo-Tethys Sea combined with cooling of global ocean temperatures might reduce the moisture supply to continental interiors, leading to cooling and aridification in the Asian interior. Mineralogy and geochemical data also reveal that the Tarim basin has undergone multi-phases tectonic and climatic changes since the late Oligocene. Clay mineralogy and geochemical proxy data from these sedimentary archives can shed light on climate and tectonic trends via changes in rock weathering and uplift rates. An abrupt mineralogical shift at 17 Ma±1 Myr in the Miocene Qimugan section in the northwestern part of the QTP was observed. The rapid shift involves increasing trends of smectite, chlorite, and calcite contents. These trends were ascribed to changing source rocks due to uplift of the northern part of the QTP leading to exposures of younger intrusive bodies and older gneisses, and schists, and Paleogene smectite- and carbonate-rich source rocks. These uplifts potentially caused regional aridification reducing chemical weathering as indicated from clay morphologic evidence and increase of chlorite. The dating is indirect via magnetostratigraphically-dated ostracod biostratigraphy and detrital zircon chronology and currently not good enough to compare the shift accurately in time with the onset of the global Middle Miocene Climate Optimum (MMCO) at 16.5 Ma. Nevertheless, regional tectonics seem to have dominated over global climate as the warmer MMCO and intensify the uplift of the arrounding Tarim, leading to the aridification. A subsequent change occurred in the latest Miocene at 6.5±0.5 Ma characterized by increases of calcite, kaolinite, chlorite, and smectite contents and decreases of illite content. The increase of kaolinite and change of clay morphology from physical-dominant to chemical-dominant weathering are not consistent with the climatic condition which is increasing aridification. This step again indicates a provenance change, probably related to simulataneous onset of eolian dune building in the Tarim Basin. The sediment source after 6.5±0.5 Ma is likely similar to that of the eolian dunes, which accumulate from the sources surrounding the Tarim Basin.The Qaidam basin located in the northern part of the QTP with continuous Cenzoic sedimentary sequence provides an ideal carrier to access the climatic and tectonic evolution of the northern QTP. Clay mineralogy and its palaeoclimatic interpretation of the early-Eocene (-53.3-49.70 Ma) sediments at Lulehe Formation, Qaidam basin, were investigated using optical microscopy, SEM, and XRD. The interval of ～53.3-49.70 Ma, including the early-Eocene climate optimum (EECO) with isotopic events, was the transition period of "greenhouse" to "icehouse". Climate changes during the episode were documented in the sediments and were expressed by the proportion of clay species and clay indices, as well as by the proportion of non-clay minerals, gypsum, halite and calcite. Our results suggest that a warm and humid climate prevailed over the period ～53.3-52.90 Ma, followed by a warm and seasonally dry and humid climate in the period ～52.90-51.0 Ma and a subsequently warm and humid climate in the period ～51.0-49.70 Ma. The increases of smectite and mixed layer illite-smectite, values of illite crystallinity reveal three warmer and more humid intervals at 52.7,51.0 and 50.5 Ma. The climate evolution in the Qaidam Basin during the period derived from the clay mineralogical study is in good agreement with the early Eocene global climate change, and the warm and seasonally dry and humid episode in the early Eocene in Qaidam basin is a regional response to the global early-Eocene climate optimum. To better assess the climatic evolution related to the uplift of the northern part of QTP, long-term clay mineralogy and bulk mineral composition of Tertiary sediments in Qaidam were investigated. Climate change in Qaidam since ～53.5 Ma could be divided into four stages:a warm and seasonally arid climate between ～53.5 and 40 Ma, a cold and arid climate from ～40 to 26 Ma, a warm and humid climate between ～26 and 13.5 Ma, and a much colder and arid climate from ～13.5 to 2.5 Ma, respectively. The cool and dry climate documented at-40Ma could be caused by the retreat of the Neo-Tethys Sea, while the warm and humid climate during-26-13.5 Ma is likely the reginal response of the global late Oligocene climate optimum and middle Miocene climate optimum. The illite crystallinity and sedimentary facies suggested that uplift events took place around>52-50,-40-38,-26-15,～10-8, and<5 Ma in the Qaidam region, respectively, which are the responses of mutli-phase uplifts of the northern QTP. The climatic model reveals the climatic condition in Qaidam Basin could have been mainly controlled by global climate prior to 13.5 Ma. As the Tibetan Plateau reached a significant elevation by ～13.5 Ma, and the climate cycles of the East Asian monsoon might add additional influence.The Gonghe Basin located in the northeastern QTP began to accumulate since the early Miocene (-22Ma) and is composed of a continuous river-lacustrine sedimentary sequence. XRD and Fourier infrared absorption spectroscopy (FTIR) were conducted to deepen our research on specific species and spectral characteristics of swelling clay minerals in the late Miocene sandstones in Xinghai, Qinghai province. XRD results show that swelling clay minerals are dominant clay minerals in the sandstones, which can be up to 97% in percentage. XRD patterns show d060 reflection of the samples occur both remarkably at 1.534 A and 1.498 A, indicating the samples contain physical mixtures of trioctahedral and dioctahedral swelling clay minerals. Further treatment of Li-300℃ heat and glycerol saturation show the swelling clay minerals collapse to 9.3-9.9A with a partial expansion to～18 A. This indicates the swelling clay minerals dominate montmorillonite and contain minor saponite. FTIR results show the samples are composed of a kind of phyllo silicate with absorbed and structural water, which is in agreement with the results of XRD. Absorbed peaks at 913 cm-1,842 cm-1,880 cm-1, corresponding to OH associated with Al-Al, Al-Mg, and Al-Fe pairs, further indicate the minerals are mainly dioctahedron in structure. Meanwhile, absorbed peaks at 625 cm-1 and 519 cm-1, corresponding to coupled Si-O and Al-O-Si deformation, indicates parts of Si is replaced by Al in tetrahedron. The spectral characteristics of the samples are against the presence of beidellite and nontronite based on the results of XRD and FTIR, while demonstrating an existence of montmorillonite. This study to distinguish the specific species of swelling clay minerals in clay minerals would be of great importance when using clay mineralogy to interpret provenance and climatic information. Long-term clay mineralogical proxy from the Gonghe Basin sediments, spanning 22-7 Ma, show that five distinct clay mineral assemblages can be divided, which indicates the paleoclimate has undergone five-stage evolutions:1)-22-19.7Ma, kaolinite-illite assemblage, refleting climate dominatantly warm and humid; 2) 19.7-19.0Ma, illite-kaolinite-smectite assemblage, climate dominatantly warm and humid, while drier than the preious stage; 3) 19.0-12.6Ma, smectite-illite assemblage, climate dominatantly seasonal dry; 4) 12.6-9.8Ma, smectite-illite assemblage, smectite lower than previous stage, climate dominatantly seasonal dry and much colder; 5) 9.8-～7Ma, dominatant smectite, climate dominatantly much drier and colder. Scanning electron microscopy resluts show after 19.7Ma, authigenic smectite initiated to new-form, and exhibited incipient honeycomb aggregation. After～19 Ma, the authigenic smectite presents flake-shape and honeycomb structure. The authigenic smectite has close relationship with sandstone, while in the medium sandstone pumping the maximum, indicating the porosity is responsible for the growth of smectite. Climate change adjusting the ion concertration of the porosity water could cause the neoformation of smectite. Climate changes to seasonal dry after 19 Ma, which could favore the neoformation of smectite. Signifcant uplift of Qinghai-Tibetan Plateau after 19 Ma resulting in the climate model changing from a’planetary’subtropical aridity zonal pattern to a monsoon-dominated "inland" aridity pattern is likely the driving mechanism leading to the aridification in Asia.Combined with the clay mineralogical and geochemical results from the long-term representative sections in different parts of the northerm QTP, climate condition of the northerm QTP was a’planetary’subtropical aridity zonal pattern before the late Oligocene. Sustaining extrusion between Indian and Eurasian plates resulted in a northward drifting of the northern part of QTP during the middle-late Eocene, then enlarging the area controlled by the subtropical aridity, which expanded the arid area. Further extrusion caused the retreat of the Neo-Tethys Sea, which combined with the subsequent global Eocene-Oligocene cooling decreasing the moist input from westerlies to Asian inland. This mechanism significantly caused a cooling and drying during the Eocene-Oligocene transition in the Tarim Basin as suggested by distinct declines of smectite and kaolinite. Climate cooling and drying in the Qaidam basin at ～40 Ma possibly linked with the retreat of the Neo-Tethys Sea, while no climate change at Eocene-Oligocene transition was observed. After the late Oligocene, the’planetary’ subtropical aridity zonal pattern changed to the monsoon-dominated "inland" aridity pattern as indicated from the cooling and drying at～19 Ma in the Gonghe Basin and～17 Ma in the Tarim basin, respectively. However, during this period, the climate was still interrupted by the global climatic change as strong signals revealing the late Oligocene and middle Miocene climate optimum were recorded in the Qaidam basin. During the middle and late Miocene, the accelerated uplift enhanced the intensity of the Asian monsoon, leading to major uplift stages linking well with the intensification of the Asian monsoon.